I could be wrong, but I think the “erase” feature assumes that the internal drive has been encrypted in real time by a T2 security chip or an Apple Silicon chip. Instead of actually erasing everything on the drive, it tells the Mac to forget the encryption keys needed to make sense out of the recorded data.
On a Mac without a T2 chip or an Apple Silicon chip, the internal drive wouldn’t be encrypted at such a low level, and you couldn’t just tell the system to forget an encryption key as a means of “erasing” the disk.